This invention relates to a preparation for epidermis containing kojic acid and/or its derivative, in which kojic acid and/or its derivative is stable with time and which gives a pleasant feel upon application to the skin and, more particularly, to a preparation for epidermis containing kojic acid and/or its derivative, which further contains a surfactant having a specific HLB value. The surfactant serves to depress coloration or decomposition of kojic acid and/or its derivative and improve application feel of the preparation.
As typical forms of preparation for epidermis, there are illustrated O/W (oil-in-water) emulsions and W/O (water-in-oil) emulsions, which are different from each other in water-to-oil composition ratio and physical properties but are both homogeneous preparations wherein oil phase or aqueous phase are stably emulsified and dispersed with the aid of a surfactant.
O/W Emulsions and W/O emulsions are greatly different from each other in application feel and physical properties. That is, O/W emulsions well fit to the skin and are less sticky, thus providing favorable application feel, but spoil stability of water-soluble ingredients. Hence, O/W emulsions have problems in designing preparation form.
On the other hand, W/O emulsions provide excellent skin-protecting effect and ensure lasting effectiveness of the ingredient, but poorly fit to the skin and are more sticky. With respect to physical properties, W/O emulsions are liable to undergo oil phase separation at lower or higher temperatures, thus being difficult to obtain emulsion stability. Therefore, there has been required a large amount of surfactant or a stabilizing aid which, in turn, causes safety problem for skin.
Kojic acid and its derivatives the inventor has long studied are known as useful agents having various excellent properties, as disclosed in Japanese Unexamined Patent Publication No. S55-157509, Japanese Examined Patent Publication No. S56-18569, S58-22151, S58-22152, S58-34446, S60-7961, S60-9722 and S60-10005, Japanese Unexamined Patent Publication No. S60-137253, Japanese Examined Patent Publication No. S61-10447 and S61-60801, Japanese Unexamined Patent Publication No. S62-5909, Japanese Examined Patent Publication No. S62-3820 and S63-27322, Japanese Unexamined Patent Publication No. H1-132502 and Japanese Examined Patent Publication No. H5-30422.
However, kojic acid and its derivatives (hereinafter these being in some cases merely referred to as "kojic acids") are also known as agents which themselves have difficulty in acquiring stability. Particularly when the kojic acids are incorporated in the aforementioned O/W emulsion or W/O emulsion, it requires a highly sophisticated technique to design a proper formulation. Hence, it has been a pressing subject with respect to formation of a preparation containing the kojic acids to develop a technique which provides the kojic acids-containing prepatation with enough stability to stand severe distributive machinery without giving unpleasant feeling upon application thereof to skin.
In the case of compounding the kojic acids in O/W emulsions wherein kojic acid dissolves in the outer aqueous phase, they are under the condition of being likely to be exposed to ultraviolet light which can be an external cause of their coloration or decomposition. Thus, it has been conducted to compound an ultraviolet light absorbent in a proper amount for depressing damages by irradiation with ultraviolet light. Examples thereof are illustrated in, for example, Japanese Unexamined Patent Publication No. S62-108804 and S64-83008 and Japanese Examined Patent Publication No. H4-46924.
However, coloration of kojic acid is caused not only by irradiation with ultraviolet light but by heating and the like. Hence, the above-described techniques are still insufficient, and there have been attempted to use other stabilizing agents. Such techniques are described in, for example, Japanese Examined Patent Publication No. S63-24968 and H2-27963, Japanese Unexamined Patent Publication No. H3-101609, H3-188011 and H3-193712.
In the case of incorporating kojic acid in a W/O emulsion wherein kojic acid dissolves in the inner aqueous phase, coloration or decomposition of kojic acid due to ultraviolet light less takes place in comparison with an O/W emulsion. But, in some cases, kojic acid undergoes change with time due to mutual action with base ingredients. In order to depress such change, there have been used various specific additives as described in, for example, Japanese Unexamined Patent Publication No. H2-28105.
The above-described prior art is effective to some extent but fails to completely solve the problems.
Since both type emulsions still involve the problem of coloration or decomposition to varying degrees of kojic acid with time and the problem of not providing various choice of application feel, preparations of a formulation not containing large amounts of various stabilizing agents and yet being capable of stabilizing kojic acid and easily providing desired application feel have been desired.